Problem: Some old NiMH cordless drill batteries are bad. Replacement batteries are hard to come by or annoyingly expensive.Solution: Buy some NiMH batteries and make my own -- reusing the plastic casing of the originals. Need a low area of heat way of welding these batteries together to make a form-factor compatible battery pack. So I need a spot welder.

I have in my possession a Microwave Oven Transformer(MOV) and some knowledge.

My thoughts are to build the first (Easiest) but making two "Wands" for hand placing the compression points instead of the "punch" style...

I don't have a lot of experience with either style (Capacitive Discharge or Transformer/high-current).I would THINK a Capacitive Discharge spot welder would be more effective in creating a quick, local weld with minimal heat dissipation into the batteries, but both seem like they'd do the trick...

Does the Hacker Factory already have a low-heat, quick/localized discharge weld source? Would there be a use beyond my battery pack if I made one of these?

A spot welder is probably the one piece of welding equipment we don't have right now. And it would come in handy for more than battery packs -- it's not easy to MIG-weld thinner metals without blowing clean through.

Though I have not yet become a TC Maker member or used the facilities yet (just stopped by on one Wednesday night to see what it's all about), I'll keep this thread updated. It's been kind of a side project to the FIRST Robotics work that's been taking up my time.

At this point I've got the MOV transformer's secondary windings removed and replaced with a 4ft piece of stranded 4AWG wire wind.

The 4AWG wire is crimped on both ends with some fairly beefy ring lugs. I also have two 1ft. lengths of solid 4AWG copper wire (grounding rod) that I want to join with the 4AWG stranded.

That's where it's at so far. I have not plugged it in and observed/measured current draw or anything yet.

I've finished revision 0.1 of this project. It is lacking (at least) three things that I feel would make it a practical tool:

An inline circuit breaker

A means of controlling the current/power output (variable resistor on primary winding side)?

A hands-free means of turning the surge/power on and off (foot switch interrupt on primary coil side of transformer.

That being said, it certainly does output plenty of current. I haven't measured the current flow when shorted, but it's shown itself to be enough to vaporize thin pieces of metal. A means of controlling output should help dial in the correct output for various jobs.

metis wrote:well as boltz said, an scr isn't a bad option (but that's a tiny one in the link, you should see some vintage theatre dimming gear)

So I think what you're proposing is having an SCR inline with the AC waveform on the secondary winding?

In reading up on theory around an SCR, I seem to see that a nice medium between cheap, one SCR control circuitry is thus:effectively losing half the waveform, but then it's a one-way current path through the two metal pieces... Might that be better in some way?I'm kind of a small signal circuit guy. This power electronics stuff is a bit out of my scale of thought (i.e. not very intuitive).

filed under: electronics things that i don't understand but have replaced.

i've had scr's explained to me several times, and all i really got out of it was it's a part that's going to fail at some point in analog theatrical dimming systems, and isn't bad to replace. similarly, i know that they're used in home brew transformer spot welders, but the theory behind it is magic blue smoke to me.

the big ones though... i've replaced ones bigger than a roll of silver dollars in theatre gear, (mostly 4k+ dimmers) but seen much bigger size wise on older, scary stuff.